Tucked into the Canadian Rockies is a small limestone quarry formed 530 million years ago called the Burgess Shale. Discovered early in the century, the shale holds the remains of an ancient sea that nurtured more varities of life than can be found in all of our modern oceans.

Darwinian theory says that animals living so long ago were necessarily simple in design and limited in scope. But more recent interpretations unexpectedly reveal the great diversity locked in the shale.

Explosive stuff, for it blasts the belief that the history of life has been a broadening of options and challenges the idea that humans crown the evolutionary process.

Stephen Jay Gould advocates the role played in this process by chance. Things could easily have gone differently. It makes the reader wonder what might have been, and lets each of us provide our own answer.